In mass spectrometry it is sometimes required to control an operational parameter of the instrument in order to optimise the acquisition of data from species of interest. However, it is often not possible to determine the optimal acquisition parameters for the species of interest in advance of an experiment. Also, different parameters may be optimal for different species of interest.
It is known to analyse ions and accumulate data whilst the value of an operational parameter, such as collision energy, is being ramped. This enables the operational parameter to be optimised for each of the species during a part of the acquisition period and hence the data obtained can be used to generate a spectrum having contributions from optimal values for each species. However, the operational parameter will not be optimised for any given species during much of the acquisition period and hence the spectrum necessarily includes significant contributions from non-optimal values.
For example, when using an analytical filter such as a quadruple mass filter in which a single species is transmitted at any one filter setting, instrument conditions may be optimised for each mass to charge ratio value independently. One example is the optimisation of DC potentials on a DC focussing element for best ion transmission in an atmospheric pressure ion source. In many cases there is a compromise of selecting between DC potentials that optimise ion focussing so that ion transmission is improved and DC potentials that cause ion fragmentation due to collisions with background gas. In some cases, this fragmentation may be desirable and in others it may reduce the transmission of a selected ion. Such a DC focussing element may be termed Source Fragmentation Voltage (SFV) or “Cone Voltage”.
In contrast, a time of flight (TOF) instrument transmits the whole mass range at once, and so the SFV cannot be optimised by for every mass to charge ratio being analysed at the same time. The state of art is to either fix the SFV for an experiment or to scan the operational voltage over a range while acquiring multiple time of flight spectra and then average the resultant spectra. This leads to poorly optimised signals and for a given mass to charge ratio the SFV may give poor sensitivity and/or promote fragmentation of other masses, causing increased noise.
Although a couple of examples have been provided above, many other operational parameters would benefit from being optimised according to the nature of each spectral species. The current state of the art can only result in a best compromise value of the operational parameter for the species which are simultaneously analysed.
It is therefore desired to provide an improved method of mass spectrometry or ion mobility spectrometry, and an improved mass spectrometer.